Electron-multiplier system



April 23, 1940. R. 1.. SNYDER, JR

ELECTRON-MULTIPLIER SYSTEM Filed July 30, 1958 attorney Patented Apr. 23, 1940 ELEGTRON-MULTIPIJER SYSTEM Richard L. Snyder, 51%, Glassboro, N. 3., assignor I to Radio Corporation of America, a corporation of Delaware Application July 30, 1938, Serial No. 222,186

l Claims.

My invention relates to electron discharge devices, particularly to electron multipliers and has special reference to the provision of improvements in the construction and operation of high gain photo-actuated electron multipliers.

One of the principal difiiculties encountered in the use of electron multipliers is that of maintaining the gain of the device at a constant value irrespective of changes in the tube characteristics during operation and in spite of variations in the line voltage.

Accordingly, an object of my invention is to obviate the above described and other disadvantages incident to the use of electron multipliers.

Another object of my invention is to provide an improved electron multiplier and one which lends itself readily to the regulation of its gain.

Another object of my present inventionis to provide a novel method of regulating the gain of an electron discharge device.

Other objects and advantages together with certain details of construction and operation will be apparent and the invention itself will be best understood by reference to the following specification and to the accompanying drawing wherein the single figure is a partly diagrammatic perspective view of a multistage electron multiplier energized and operated in accordance with the principle of the invention.

3 My invention is predicated upon my discovery that when electron current passes through an electron multiplier (for example, of the type disclosed in copending application Serial No. 205,672, to Pike et al., filed July '7, 1938) there is very little transverse spreading of the electron stream. That is to say, if a spot of light is focused on one side of the center of a photo-cathode, the anode will receive the multiplied electron current only on that side. Similarly, if two 40 beams of light are directed upon the cathode, one

on one side of its center and one on the other,

two discrete electron streams will be generated.

The separation between the streams will be maintained throughout the tube and if two anodes are provided two discrete output currents are produced.

Accordingly, my present invention contemplatesand its practice provides an electron discharge device having an auxiliary anode. A to modulated and an unmodulated beam of light are provided for generating two electron streams which eventually impinge upon the separate anodes. Small changes in the unmodulated current present on the auxiliary anode are utilized to vary the actuating voltage applied to a "con- (Cl. 250-4L5) trol electrode whereby to oppose the original change. The electrode thus controlled may comprise an intermediate multiplying electrode and the controlling voltage applied thereto, from the auxiliary anode, may operate to defocus or refocus the main electron stream, whereby the number of electrons impinging upon the main anode, and hence the modulated output current of the device, is regulated.

In carrying the invention into efiect, I may employ an electron-multiplier similar to that described in the above mentioned application Serial No. 205,672 but modified to include a bipart output electrode assembly. An electronmultiplier which has been thus modified, is shown in the accompanying drawing.

The electron multiplier of the drawing comprises a highly evacuated envelope T, which, for

purposes of this description, may be considered to contain a longitudinal reference axis A-A. Suitably mounted adjacent one end of the axis AA are two anodes, B, B, respectively, which are arranged in parallel relation on opposite sides of the axis. Mounted adjacent the other end of the axis AA is an oversize photosensitive cathode C. A pair of sets of curved substantially lL-shape multiplying (i. e., secondary electron emissive) electrodes are mounted on opposite sides of the reference axis between the cathode and bi-part anode. The electrodes of the "upper set are odd-numbered l, 3, ii, i and 9, and those of the lower set are even numbered 2, t, t and t.

The long leg of each L-shaped multiplying electrode, except electrode l, extends across the axis AA with its free end inclined toward the cathode. The short leg of each of the said US extend in the direction of the anodes and each terminates at a point substantially equally distant from the said reference axis. As more fully described in the previously identified Pike et al. application, the contour of, the cathode C preferably differs from that of the first multiplying electrode l and the contour of the electrode l, preferably, but not necessarily, diiiers from the contour of the other multiplying electrodes. However, all of the electron emissive electrodes are of cylindrical curved construction and are mounted with the generatrices of their surfaces normal to a plane containing the reference axis A-A. The hollow box-like metal member S which forms a continuation of the surface of the cathode is designed to prevent wall charges (on the glass) from affecting the motion of the phories connected resistors R, RI and a group of resistors TI, 12 and 13, which areconnected in parallel with R and RI, is provided. The anodes B, B are normally maintained at approximately the same voltage (lllV) by the potential drop developed, respectively, across resistors 1'! and RI. The cathode C is connected to the negative terminal of the direct current source by a lead and the first multiplying electrode is connected to a point on R, indicated at IV somewhat more positive than the cathode. The other electrodes 2 to 9, inclusive, in the order of their numbers, are shown connected to successively more positive points on the voltage divider. The reference characters IV, 2V, 3V, etc., given to the several points on the resistors R and 1' will be understood to indicate that the voltage drop between the given electrode and the cathode is the designated whole number multiple of the drop existing between the cathode and the first multiplying electrode l. Thus, where the potential drop between the first multiplying electrode I and the cathode C is 100 volts, the drop between electrodes 3 and C should preferably be 300 volts, that between the output electrode B and the cathode, 1000 volts.

In accordance with my invention, two separate light beams are directed upon the photosensitive surface of the cathode C to there give rise to two discrete streams of photo-electrons. The separate light beams may be derived from a common source exemplified by a lamp L, in which case a lens M may receive some of the light directly,

' and another lens N receive some of the light indirectly through a. prism P. The lenses M and N are preferably so arranged with respect to the cathode C that the beams are directed to points which are laterally well spaced from each other on the photosensitive surface of C. One of the light beams, in this case the beam from the lens M, may be modulated as by passing the light through a moving picture film F which is suitably interposed in the space between the lens M and the photosensitive surface of the cathode C. The quantity of electrons released [by the impress of the modulated light beam upon the cathode will, of course, be determined by the instantaneous intensity of that beam when it has passed through the film. On the other hand,

tron streams striking electrode I, on opposite sides of its center, will cause the emission of two streams of secondary electrons, the number of electrons in each stream being dependent upon the intensity of the impinging primary stream.

The next electrode in point of electron travel is the second lower electrode 2. The trajectories of the secondary electrons constituting the discrete electron streams from the first multiplying electrode 1 is such that the two streams impinge upon spaced areas upon the cupped surface of the second multiplying electrode 2. Here again, a multiplication by reason of secondary emission is secured and this is repeated in any desired number of stages until the amplified and still separate modulated and unmodulated streams of secondary electrons are collected upon the inclined surfaces of the anodes B, Bl, respectively.

It will be observed from an inspection of the drawing that voltages applied to the multiplying electrode 6 and to the auxiliary anode Bl are derived from that portion of the voltage divider which is constituted by resistors rl, 1'2 and T3. Thus, any change in the electron current to the auxiliary anode Bl will give rise to a counter change in the potential applied to electrode 6. This is so because the electron current to and from electrode 6 is negligible when compared to the current flowing through resistors r and 1- and because the current flowing through electrode BI, when the cathode is illuminated, is large compared to the current drawn through resistor rl when the cathode is unilluminated.

Therefore, an increase or decrease in the current to Bi when the device is in operation will give rise to a similar change in the voltage across TI and an opposite or compensating change in the voltage drop between the negative terminal of the voltage divider and the point to which electrode 6 is connected. If the point :t-GV to which electrode 6 is connected is properly chosen (between the values of 5V and IV) a change in the control voltage applied thereto will result in a change in the focus or refocusing of the electron streams required to compensate for changes in the focusing which occurs by reason of undesired variations in the gain of the device. Thus, the modulated current which flows from the main anode B to the output impedance Z and meter M is stabilized.

Various modifications of the invention will suggest themselves to those skilled in the art. It is to be understood, therefore, that the foregoing is to be interpreted as illustrative and not in a limiting sense except as required by the prior art and the spirit of the appended claims.

What is claimed is:

1. Method of controlling the gain of an electron-multiplier device which comprises generating two discrete electron streams in said device, directing said discrete electron streams to different areas of a common multiplying electrode, separately collecting the resulting multiplied electrons comprising each of said streams, and then controlling the passage of one of said electronstreams through said device by the current resulting from the collection of the electrons comprising the other of said electron streams.

2. Method of controlling the gain of an electron-multiplier device which comprises simultaneously producing a signal bearing and discrete non-signal bearing stream of electrons in said device, directing said discrete electron streams to difierent areas of a common multiplying electrode, separately collecting the resulting multiplied electrons comprising each of said streams and then controlling the passage of said signal bearing electron stream through said device by the current resulting from the collection 01. the electrons comprising said non-signal bearing electron stream.

3. Method of controlling the gain of an electron-multiplier device which comprises generating two discrete electron streams in said device, directing said discrete electron streams to different areas of a common multiplying electrode, separately collecting the resulting multiplied electrons comprising each of said streams and then controlling the multiplication of the electrons comprising one of said streams by the current resulting from the collection of the electrons in the other of said streams.

4. Method of controlling the gain of a photosensitive electron-multiplier device which comprises producing two separate light beams, im-

pressing a signal on one of said light beams, causing the signal bearing and the non-signal bearing light 'beams to respectively produce a signal bearing and a non-signal bearing elec- 0 means ror generating a plurality of discrete electron streams in said envelope, means for directing said discrete electron streams to diiferent areas upon said multiplying electrode and thence to said separate collecting electrodes, and means responsive to the current resulting from the collection of the electrons comprising one of said electron streams for controlling the passage of the other of said electron streams through said device.

6. The combination with an electron multiplier comprising a cathode, a multiplying electrode and a plurality of separate collecting electrodes mounted within an evacuated envelope, of means including said cathode for simultaneously gen-- erating a signal bearing and a discrete non-signal bearing electron stream in said envelope, means for directing said discrete electron streams to difierent areas of said multiplying electrode and thence to said separate collecting electrodes,

and means responsive to the curfiant resulting from the collection of the electrons comprising said non-signal bearing stream for controlling the passage or said signal bearing electron stream through saidenvelope device.

RICHARDL. SNYDER, JR. I 

